Chemical vapor deposition (CVD) has been extensively used for preparation of films and coatings in semiconductor processing. CVD is a favored deposition process in many respects, for example, because of its ability to provide highly conformal and high quality films at relatively fast processing times. Further, CVD is beneficial in coating substrates of irregular shapes, including the provision of highly conformable films even with respect to deep contacts and other openings.
Metalorganic chemical vapor deposition (MOCVD), wherein organometallic precursors are utilized for depositing films, is highly dependent upon suitable organometallic precursors. Generally, gases or volatile liquid precursors are utilized for such processes as they can be easily delivered to the process as a vaporized material. Solid precursors are generally considered to be a poor choice due to difficulty of vaporizing, i.e., subliming, a solid at a controlled rate. However, there are many off-the-shelf solid precursors available, particularly solid organometallic precursors, which if they could be delivered effectively and reproducibly, could be used for CVD processes. Further, solid precursors are particularly useful in deposition of metal-based films, such as, for example, metal nitrides and metal silicides.
Typical CVD processes generally employ precursor sources in vaporization chambers that are separated from the process chamber wherein the deposition surface or wafer is located. For example, liquid precursors are typically placed in bubblers and heated to a temperature at which they vaporize, and the vaporized liquid precursor material is then transported by a carrier gas passing over the bubbler or through the liquid precursor. The vapors are then swept through a gas line to the process chamber for depositing a CVD film on a surface therein. Many techniques have been developed to precisely control this process. For example, the amount of material transported to the process chamber can be precisely controlled by the temperature of the liquid precursor reservoir and by the flow of the carrier gas bubbled through or passed over the reservoir.
However, similar techniques for solid precursors are not adequate for providing a vaporized solid precursor suitable for depositing CVD films. For illustration, similar techniques may include bulk sublimation of the solid precursor with transport of the vaporized solid precursor to the process chamber using a carrier gas in much the same way as the vaporized liquid precursor is transported. However, it is difficult to vaporize solid precursor at controlled rates such that a reproducible flow of vaporized solid precursor can be delivered to the process chamber.
Lack of control of solid precursor sublimation is, at least in part, due to the changing surface area of the bulk solid precursor as it is vaporized. Such a changing surface area when the bulk solid precursor is exposed to sublimation temperatures produces a continuously changing rate of vaporization, particularly for thermally sensitive compounds. This ever changing rate of vaporization results in a continuously changing and non-reproducible flow of vaporized solid precursor delivered for deposition in the process chamber. As a result, film growth rate and composition of such films on wafers in the process chamber deposited using such vaporized solid precursors cannot be controlled adequately and effectively. Therefore, it is important to precisely control the exposure of the solid precursors to elevated temperatures without bulk decomposition of the solid precursor material.
In addition to solid precursors being difficult to deliver to process chambers at a controllable and reproducible rate, liquid source materials for CVD are also, in many circumstances, difficult to deliver to process chambers. Liquid source materials have become widely utilized, at least in part due to the fact that in many circumstances CVD cannot be accomplished using compounds that are gaseous at ambient conditions. For example, liquid sources utilized in CVD include such sources as tetraethoxysilane (TEOS) used as a source of silicon to deposit silicon dioxide films, sources for use in deposition of titanium nitride films, and sources for depositing metal oxides (for example, tantalum oxide, niobium oxide, aluminum oxide, and titanium oxide), ferroelectric oxides, copper, and aluminum. Liquid sources used for doping by diffusion are typically organic sources, such as, for example, phosphorus oxychloride, phosphorus tribromide, phosphorus trichloride, and boron tribromide. Further, for depositing doped films by CVD (e.g., borophosphosilicate glass, borosilicate glass, phosphosilicate glass), common liquid sources include, for example, triethylborate, triethylphosphate, triethylphosphite, triisopropylborate, trimethylborate, trimethylphosphate, and trimethylphosphite. The liquid precursors listed above are listed for illustration only and there are many other liquid precursors too numerous to list and for which the present invention is applicable.
Liquid sources are so named because their vapor pressures are so low that they are liquids at room temperature. However, some materials, such as boron trichloride, have fairly high vapor pressures and are only barely in the liquid state at room temperature. The lower the material's vapor pressure, the more difficult it is to deliver to a CVD reactor or processing chamber. Many liquid sources can be delivered with existing bubbler technology where a carrier gas, typically nitrogen, is bubbled through the liquid to sweep some of the liquid source molecules into the processing chamber. Other liquid precursors, such as precursors for deposition of metal oxide films, due to their low vapor pressures, cannot be delivered with sufficient reproducibility with such bubbler delivery systems, particularly in device applications with small dimensions. For example, in such cases, bubbler delivery systems are not effective due to the fact that the flow of the liquid precursor is indirectly controlled via control of a carrier gas flow bubbled through the liquid precursor. Further, bubbler systems also have problems in delivering materials with very low vapor pressures which tend to condense or decompose near normal temperatures required for vaporization between the source of the liquid precursor and the processing chamber used for CVD, i.e., condense or decompose in a vaporization chamber prior to reaching the processing chamber.
One alternative to conventional bubbler technology is to provide a liquid precursor, such as a organometallic precursor, into a processing chamber utilizing an ultrasonic piezoelectrically driven nozzle which atomizes the liquid precursor and delivers a mist of droplets to the processing chamber. Further, in conventional systems, where liquid precursors are delivered to a vaporizer using mist generation, vaporization is typically carried out by contact with heated surfaces and then a carrier gas is used to deliver the vaporized liquid precursor to the processing chamber. However, such vaporizing devices for delivery systems suffer from the disadvantage of decomposition of the liquid precursors upon contact with the hot surfaces, or incomplete vaporization, which also yields inconsistent films grown under CVD conditions. For example, such decomposition may occur at the walls of a vaporization chamber.
For the above reasons, there is a need in the art for a vapor delivery system for delivering solid CVD precursors in a CVD process at a highly controllable rate and without bulk decomposition of a solid precursor material during vaporization. Further, there is also a need in the art to provide highly reproducible vaporization of liquid CVD precursors without decomposition of liquid precursors on the walls of, for example, a vaporization chamber. The present invention provides a vaporization apparatus and method, along with a system for use thereof, which overcomes such problems as described above and others that will be readily apparent to one skilled in the art from the description of the present invention below.